1. Field of the Invention
The present invention relates to DC/DC converters. More particularly, the present invention relates to DC/DC converters that prevent undesired effects when spike noise is generated within the circuit.
2. Description of the Related Art
A DC/DC converter taught by Japanese Laid-Open Publication No. 8-289535 is shown in FIG. 6 and includes a control circuit 1, a switching element 2, such as an MOS type electric field effect transistor (MOSFET), a coil L, a current sensing resistance Rsens, a capacitor C1, a differential amplifier 4 and a comparator 5. The comparator 5 outputs a xe2x80x9clowxe2x80x9d signal [L] when a voltage drop VR across the current sensing resistor Rsens caused by current IL is greater than the reference voltage VS. The comparator 5 outputs a xe2x80x9chighxe2x80x9d signal [H] when the voltage drop VR is less than the reference voltage VS.
When the output of the comparator 5 is [H], the control circuit 1 communicates a control signal to the gate of the switching element 2 with a duty ratio that is responsive to the difference between voltage V1 (obtained by dividing the output voltage Vout) and reference voltage Vref. On the other hand, when the output of the comparator 5 is [L], the control circuit 1 outputs a control signal that turns off the switching element 2.
In a voltage controlled DC/DC converter, the reference voltage VS is set to a predetermined value. Therefore, the comparator 5 operates such that the current IL is limited to a predetermined upper value regardless of the voltage difference between V1 and Vref.
On the other hand, in a current controlled DC/DC converter, the reference voltage VS is adjusted in compliance with the voltage difference between V1 and Vref. For this reason, the comparator 5 causes a large current to flow when the voltage difference is large, and operates so that the current is suppressed to a small current when the voltage difference is small.
As discussed further below, however, such DC/DC converters are susceptible to undesired effects due to spike noise generated by the coil when the switching element is switched. Therefore, it is an object of the present invention to provide an improved DC/DC converter.
In one aspect of the present invention, a DC/DC converter is taught that can adequately control the output voltage regardless of any spike noise generated when a switching element is switched and without delaying the response rate of a current limiting circuit.
In one preferred means for achieving this aspect, a DC/DC converter is taught that includes a switch (or a switching element), a coil coupled to the switch, a circuit for sensing that a current sensing signal responsive to a current flowing in the coil has become equal to or greater than a predetermined current value, a control circuit for controlling the switch in response to the output signal of the current sensing circuit and a changing circuit for preventing the current sensing circuit from operating adversely due to any spike noise generated when the switch is switched. As discussed herein, a variety of embodiments can be utilized to practice this aspect of the present teachings.
DC/DC converter circuits that have a circuit to prevent spike noise from affecting the operation are highly desirable to prevent adverse operation of the DC/DC converter. In addition, the present teachings provide DC/DC converters that can operate without delays or other interruptions.
Other objects and advantages of the present invention can be more fully understood by reading the detailed description of invention and the claims with reference to the accompanying drawings.